In the present day design of power amplifiers for driving loudspeakers and the like, the common practice is to utilize an operational amplifier in the front end of the audio system. The advantages of utilizing such an operational amplifier include the permissible use of high levels of feedback. The production of very low output off-set voltages, and a relatively high input impedance. This impedance remains constant with overload and frequency variation. In addition, the use of operational amplifier provides an economic advantage since it is available in a single integrated circuit. However, as a typical integrated circuit has relatively low supply limits, a transistor gain stage must be utilized following the operational amplifier to bring the signal level up for driving the traditional emitter follower type circuits of the complementary or quas-complementary designs. In such circuit designs, it is quite common to find supply voltages at eighty volts or higher. Now the operational amplifier will remain in the linear mode as long as the output of the power amplifier follows the input signal in a nondistorted manner.
If output clipping takes place because signal conditions exceed supply voltages or V/I limiting circuits become operational due to overloads or short circuits or slew rate limiting occurs on high slewing type signals, the input amplifier will immediately generate a nonlinear signal which will cause the output of the operational amplifier to swing to either supply rail and stay there until signal conditions are again linear. Once linear operation is established, the operational amplifier must recover at its slew rate speed back to near "O" to regain control of the amplifier. During this time, there is a so-called "dead zone" which is a nonlinear type situation presenting a common problem generally referred to as transient intermodulation (T.I.M.) distortion. To minimize such signal distortion, it is necessary to prevent the operational amplifier from going to the supply rails which is a relatively difficult action to prevent.